The field of the invention is that of microlithographic patterning, especially using charged particle beam projection systems.
In the field of lithography a reticle may contain stencil shapes, sometimes referred to as xe2x80x9cdoughnutxe2x80x9d shapes, that are open areas having a xe2x80x9cclosedxe2x80x9d configuration that permit the passage of the exposing radiation, but contain within them areas that are required to be opaque to the exposing radiation. A simple example is the letter xe2x80x9cOxe2x80x9d, which has an closed oval line surrounding a central area. With chrome on glass masks, there is no problem in using such configurations, because the glass substrate supports the central opaque section. If such a shape is to be used in a particle beam system that employs a stencil reticle (a reticle having xe2x80x9copaquexe2x80x9d regions that remove particle from the beam and open areas that permit the beam to pass though without scattering) there is the known problem of supporting the central area.
In the prior art, two complementary reticles were used to define stencil shapes, each reticle carrying a supported partial pattern that combined with the other pattern to define the correct shape, each reticle being exposed with the full exposure dose of electrons. Such reticles suffered from certain problems explained below.
The invention relates to a stencil mask for e-beam lithography that includes more than one set complementary sub-field reticle, each of which is exposed with one nth of the total dose.
A feature of the invention is that stencil (or xe2x80x9cdoughnutxe2x80x9d) shapes have n different patterns of struts, so that an area that is blocked in one shape is exposed in (nxe2x88x921) other shapes.
An advantageous feature of the invention is that, when used with a photoresist with sufficient contrast, the areas having (nxe2x88x921)/n exposure are not significantly different from the areas having the full exposure.
Another feature of the invention is the provision of areas of the reticle that are used several times for printing repetitive arrays.
Yet another feature of the invention is the provision of areas of the reticle that carry alternative versions; i.e. the wafer can be exposed with version A or with version B.